Coal is an organic material that is burned to produce heat for power generation and for other industrial and domestic applications. However, it has inclusions of mineral matter, which may contain undesirable elements such as sulfur and mercury. Coal combustion can thus produce large amounts of ash, fugitive dusts, mercury, and SO2 that may need to be handled properly. Therefore, run-of-mine (ROM) coals are cleaned of the mineral matter before utilization, which also helps increase combustion efficiencies and thereby reduces CO2 emissions. In general, coarse coal (50×0.15 mm) can be cleaned efficiently by exploiting the specific gravity (SG) differences between the organic matter (coal) and the inorganic mineral matter.
ROM coal can have a large variability of moisture and maximum particle size. The range of particle size spans from coarse coal to finely ground or pulverized coal. Slurries of coarse particles are easier to process and dewater than slurries of fine particles.
U.S. Patent Application Publication Nos. 2013/0111808 and 2013/0340328, now U.S. Pat. No. 9,518,241, which publications are incorporated herein by reference, disclose a process for cleaning and dewatering coal fines of less than ˜0.15 mm in size by first agglomerating the hydrophobic (or organic) coal in an aqueous slurry in the presence of a hydrophobic liquid (or oil). The hydrophobic agglomerates are then separated from the mineral matter dispersed in aqueous phase using an appropriate size-size separation method such as screening. During the process of forming the agglomerates, droplets of water are entrapped in between the hydrophobic particles constituting the agglomerates. Therefore, the hydrophobic agglomerates are broken (or destabilized) by a mechanical means to disengage (or liberate) the entrapped water droplets from coal particles. The breakage step is carried out in a hydrophobic liquid phase, so that the hydrophobic particles are readily dispersed. If the hydrophobic particles are fully dispersed, most of the water droplets are liberated from coal and fall to the bottom, forming an aqueous phase. When the water droplets are separated from coal in this manner, the hydrophilic mineral matter dispersed in them are also separated from coal.
If the hydrophobic-hydrophilic separation (HHS) process described above is carried out in a hydrophobic liquid that can form large contact angles (θ) in excess of 90° on the coal particles suspended in water, the hydrophobic liquid displaces the surface water, or the water molecules adhering on the surface of coal particles. This mechanism, known as dewatering by displacement (DbD) has been disclosed in the U.S. Patent Application Publication Nos. 2013/0111808 and 2013/0340328. These applications disclose methods of utilizing the HHS and DbD mechanisms (or processes) simultaneously to produce low-moisture and low-ash coal products from a variety of fine coal slurries that are generated from coal preparation plants. The moisture levels that can be achieved by following the steps disclosed in the aforementioned disclosures are comparable to those achieved by thermal drying, which is costly and creates environmental concerns.
When hydrophobic agglomerates are placed in a hydrophobic liquid such as n-pentane, water-in-oil emulsions are formed, with the hydrophobic particles serving as emulsion stabilizer or effectively as “solid surfactants.” The amount of energy (E) required to detach the hydrophobic particles from the oil/water interface may be estimated using the following relation (Binks, B. P., Current Opinion in Colloid and Interface Science, 7, 2002, p. 21),E=πr2γO/W(1+cos θ)  [1]where γO/W is the oil/water interfacial tension, r is the particle radius, and θ is the contact angle measured through the aqueous phase. Eq. [1] suggests that the higher the contact angle, the lower the energy required to disperse the hydrophobic particles in a hydrophobic liquid (or oil) phase. Eq. [1] suggests also that the smaller the particles, the lower the energy required to break hydrophobic agglomerates and disperse the particles in a hydrophobic liquid.
In the aforementioned U.S. patent applications, the hydrophobic liquids are chosen from shorter-chain n-alkanes and alkenes, both unbranched and branched, and cycloalkanes and cycloalkenes, with carbon numbers less than eight. These and other hydrophobic liquids such as ligroin (light naphtha), naphtha and petroleum naphtha, and mixtures thereof have contact angles (θ) greater than 90°, so that hydrophobic particles can be detached from water/oil interfaces with minimal energy expenditures and readily dispersed in the organic liquids. Furthermore, these hydrophobic liquids have low viscosities and low boiling points, so that they can be readily recovered by solid/liquid separation (e.g., pressure filtration) and by vaporization/condensation for recycle purpose. Liquid carbon dioxide (CO2) is another liquid that can be used as a hydrophobic liquid in the instant invention.
In the oil sands industry, bitumen is recovered by flotation and the recovered bitumen is further treated by dissolving the froth product in a hydrophobic liquid and subsequently removing entrained water along with the hydrophilic particles such as clay dispersed in the water droplets. During this step, water-in-oil emulsions are formed, causing a difficulty in dewatering. At present, the water-in-oil emulsions are destabilized using various chemicals. It is possible that the method and apparatus disclosed herewith may be used to destabilize the emulsions and facilitate the dewatering process.